Bull. Mater. Sci., Vol. 28, No. 2, April 2005, pp. 87–90. © Indian Academy of Sciences.
Effect of heat treatment on martensitic transformation in
Fe–12⋅⋅5%Mn–5⋅⋅5%Si–9%Cr–3⋅⋅5%Ni alloy
T KIRINDI* and M DIKICI†
Department of Computer Technology, Faculty of Education, Kirikkale University, 71450 Yahsihan, Kirikkale, Türkiye
Department of Physics, Faculty of Science and Arts, Kirikkale University, 71450 Yahsihan, Kirikkale, Türkiye
†
MS received 23 December 2004
Abstract. In this study, thermally-induced martensitic transformation (γ (fcc) → ε (hcp)) in Fe–12⋅⋅ 5%Mn–
5⋅⋅5%Si–9%Cr–3⋅⋅5%Ni (weight) alloy was studied by scanning electron microscopy (SEM) and transmission
electron microscopy (TEM). The effect of cooling rate was investigated. It was observed that fast cooled sample exhibited regular overlapping of stacking faults and ε martensite plates were formed parallel to each
other. TEM investigations showed that the orientation relationship between γ –εε phases corresponds to Shoji–
Nishiyama type orientation relationship.
Keywords.
1.
Martensitic transformation; Fe–Mn–Si–Cr–Ni; grain size; ε martensite.
Introduction
It is well known that the martensitic transformation, which
occurs in many Fe, Cu and Ti based alloys and ceramics,
is one of the most typical first-order structural diffusionless phase transition. It was widely studied to determine
its characteristics from physical, metallographical and
crystallographical viewpoints (Nishiyama 1978). Some
aspects of martensitic transformations, such as transformation temperature, crystallography and the amount of
the product martensite and its morphology, are strongly
influenced by external fields such as temperature and
uniaxial stress (Nishiyama 1978; Kakeshita et al 1999).
Among the iron based alloys, the most studied ones are
Fe–Mn and Fe–Mn–Si alloys which present non-thermoelastic martensitic transformation (Baruj et al 2000; Lee
et al 2003). The microstructure of ε martensite formed by
cooling in thermomechanically-treated Fe–Mn–Si–Cr–Ni
shape memory alloys has also been investigated (Inagaki
1992; Yang and Wayman 1992; Li et al 1999; Baruj et al
2004). The effect of Mn and Ni on the shape memory
effect in Fe–Mn–Si–Cr–Ni alloys was quantitatively investigated (Inagaki 1995; Jiang et al 1995). It is known
that the γ → ε transformation occurs by the motion of
stacking faults on alternate close packed planes of the
fcc structure. All {111} planes of the fcc structure are possible shear planes. It is well known that the crystallographic relationships between fcc and hcp structures are:
{111}γ //{0001}ε and < 1 1 0 >γ // < 1120 >ε (Bergeon et al
1998).
*Author for correspondence ([email protected])
In the present work, the effect of cooling rate on the
thermally-induced martensitic transformation in Fe–12⋅5%
Mn–5⋅5%Si–9%Cr–3⋅5%Ni alloy is investigated by SEM
and TEM.
2.
Experimental
Fe–12⋅53%Mn–5⋅35%Si–8⋅95%Cr–3⋅38%Ni alloy was
prepared by induction melting under an argon atmosphere,
using high purity iron, manganese, silicon, chromium and
nickel. The chemical composition of the alloy was determined by EDS. The arc-melted ingots were cut by a diamond cutter at room temperature. Samples obtained from
the buttons were sealed in quartz capsules. The sealed
specimens were homogenized at 1050°C for 1⋅5 h and after
homogenization one of the samples was furnace cooled
and another one was quenched in water at room temperature. The SEM specimens were prepared by conventional
mechanical polishing followed by etching with acetic
glyceregia (1 ml HCl + 10 ml acetic acid + 5 ml HNO3 + 2
drops of glycerin) (Andrade et al 1999). SEM observations were performed in a JEOL 5600 scanning electron
microscope.
From the quenched sample, several TEM specimens
were prepared. Discs of 0⋅4 mm thickness were spark cut
and thinned to 0⋅2 mm with a 800 grit emery paper. Finally, the discs were electrolytically thinned by the double-jet technique using a solution of 92% acetic acid and
8% perchloric acid at 10°C. The thinning voltage was set
at 20 V and the current was 64 mA. The TEM observations
were performed by means of a JEOL 3010 electron
microscope with an operating voltage of 300 kV with a
double tilt specimen.
87
T Kirindi and M Dikici
88
3.
Results and discussion
3.1 Effect of cooling rate on behaviour of martensitic
transformation
The effect of cooling rate on the behaviour of martensitic
transformation was investigated by comparing the microstructure of the specimens cooled in furnace or quenched
in water. Figure 1a shows scanning electron microscopic
image of Fe–12⋅5%Mn–5⋅5%Si–9%Cr–3⋅5%Ni alloy which
was furnace cooled. As seen in figure 1a, the γ austenite
appears as large domains and the transformation has not
begun. The austenite grain size was from 20 to 100 µm in
the furnace cooled sample. Figure 1b shows large domains of martensite appearing as a very tight juxtaposition of thin plates in the water quenched specimen. The
microstructure of this specimen at room temperature consists of several thermally-induced ε martensite bands
Figure 1. SEM micrographs of specimens: a. secondary
electron image (SEI) of austenite phase in furnace cooled Fe–
12⋅5%Mn–5⋅5%Si–9%Cr–3⋅5%Ni and b. back scattering electron image (BEI) of ε martensite phase which formed in the
austenite grains in the water quenched alloy.
parallel to each other and untransformed γ phase. Most
of the bands of ε martensite pass through the whole
grain and for smaller grains, usually different directions
of ε martensite bands are formed in a grain (Jiang
et al 1995).
3.2 Transmission electron microscopy observations
The TEM observations of water quenched sample revealed that the martensite plates generally formed in
whole austenite grain. Within each γ grain, a large number of widely extended overlapped stacking faults and ε
martensite bands intersecting frequently with each other
were observed (Inagaki 1995).
Figure 2 shows the presence of a large number of stacking faults, which mainly run in the stacking direction. The
stacking faults were formed in the {111}γ planes, that are
usually the known fault planes for this system (Maki and
Tsuzaki 1992; Yang and Wayman 1992). The stacking
faults end in other defects, such as grain boundaries and
dislocations. Based on the analysis of electron diffraction
pattern and the morphology of stacking faults, it was
found that the stacking faults were oriented in one of the
12 {111} <111>γ systems (Li et al 1999). The stacking
fault energy of Fe–Mn–Si based shape memory alloys is
very low, and perfect dislocations can easily split into
two 1/6 <110> partial edge dislocations, which form
stacking faults on the (111)γ planes along the <110>γ
directions (Yang and Wayman 1992).
Figure 3a shows the ε martensite plates in the water
quenched sample. These martensite bands are on the
{111}γ planes in austenite phase where stacking faults
have a {0001}ε fault plane. Figure 3b gives the diffraction
pattern of the ε plates and key diagram of this pattern.
The zone axes for austenite and martensite phases were
determined as [ 1 10]γ and [1210]ε , respectively and the
Figure 2. Typical bright field electron micrograph of Fe–
12⋅5%Mn–5⋅5%Si–9%Cr–3⋅5%Ni alloy showing widely extended
stacking faults.
Effect of heat treatment on Fe–12⋅5%Mn–5⋅5%Si–9%Cr–3⋅5%Ni alloy
89
Figure 3. a. TEM micrograph showing the formation of thermally induced ε bands and b. diffraction
pattern and indices diagram.
orientation relationship was determined as (111)γ //(0001)ε,
[ 1 10]γ // [1210]ε which corresponds to the Shoji–Nishiyama relationship (Nishiyama 1978; Yang and Wayman
1992).
water quenching. From the TEM observations and crystallographic investigations of this shape memory alloy,
the formation of ε martensite crystals obeyed a Shoji–
Nishiyama type relationship.
Acknowledgements
4.
Conclusions
In this paper, an analysis of the morphology of the ε
martensite formed in Fe–12⋅5%Mn–5⋅5%Si–9%Cr–3⋅5%
Ni shape memory alloy was carried out by using SEM
and TEM. As a consequence of furnace-cooling (slow
cooling), grains of the austenite phase with a grain size
between 20 and 100 µm were observed. However, ε martensite was observed in the austenite grain as a result of
The authors gratefully acknowledge the research grant received from Kirikkale University (Project No. 02/03–05–
03). Thanks are also due to Hauner Metallische Werkstoffe,
Germany, for sample preparation.
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